1. Field of the Invention
The present invention relates generally to nuclear medical imaging devices and more particularly relates to a method for converting SPECT projection data for different collimators.
2. Description of the Related Art
In a nuclear medicine imaging device, such as a gamma camera for obtaining either planar images or Single Photon Emission Computed Tomography (SPECT) images, a collimator is mounted to the face of the imaging device. The collimator collimates radiation (e.g., a gamma photon) which is emitted from a source within a patient before the radiation strikes a detector crystal. In FIG. 3, a block diagram is shown of an exemplary SPECT device. A radiation source 302 within an object to be imaged 304 (e.g., human body part) emits gamma photons that emanate from the object 304 and are captured by a detector 306, which converts the detected radiation into spatial projection data. A collimator 308 collimates the radiation and prevents non-orthogonal radiation from being detected. Such collimators typically include some sort of shielding, such as lead, with holes that allow radiation (e.g., photons) to pass through to the detector. In SPECT, the detector is moved about the object being imaged and acquires projection data from each of a number of different view angles. A processor 310 then constructs a 3-D SPECT image from the detected projection data using some sort of reconstruction process or algorithm. Standard reconstruction processes exist for parallel hole type collimators.
Collimators used in nuclear medicine can be parallel hole, converging (e.g., cone-beam) or diverging (e.g., fan-beam), or arbitrary hole. The geometries of different collimators give their outputs different characteristics. As a result, methods of constructing a SPECT image are typically customized for the type of collimator used during detection/collection of the image data. If data is collected with one type of collimator, but the reconstruction tool for constructing the image is for a different, second collimator, the data must be “rebinned” from spatial coordinates corresponding to the first collimator to spatial coordinates corresponding to the second collimator.
Rebinning methods exist. However, current rebinning methods do not account accurately for the effective 3D-beam angle of collimator holes and the effect of the Point Spread Function (PSF), which provides a measure of the amount of blurring of a single point due to non-ideal optics, as a function of the position of the hole on the collimator, i.e., PSF is not stationary for converging or diverging collimators. Effective angle resolution can be modeled as a Gaussian with the full-width half-maximum (FWHM) as:FWHM αHD/T (R+T);wherein HD is the hole diameter for the holes of the collimator, T is the thickness of the collimator, and R is the distance from the collimator face to the rotation center of the imaging device.
The PSF in collimator coordinates (x,y) for a point source at distance R for a parallel hole collimator is thus, stationary. That is, PSF=constant  (x,y) when R=constant. In the case of a converging or diverging collimator (e.g., fan-beam, cone-beam, etc.) the hole length L≠T, unlike the case as the parallel hole collimator. See FIG. 1. The hole (102) length L of a fan beam collimator can be calculated by: L=T/(sin α). Thus, the FWHM becomes a function of the angle α of the fan beams (i.e., angle of the holes in relation to the surface of the collimator). As a result, the point spread function, PSFfanbeam(x) α Gaussian (FWHMF (x)). And,
                    FWHM        F            ⁡              (        x        )              ⁢    α    ⁢          HD                        T          /          sin                ⁢                                  ⁢                  α          ⁡                      (            x            )                                ⁢          (              R        +                              T            /            sin                    ⁢                                          ⁢                      α            ⁡                          (              x              )                                          )        ,which significantly impacts the resolution. Similarly, with some collimators, the hole diameter is not constant for all holes and the point spread function would change accordingly.
Depending on the collimator design, resolution and sensitivity may or may not be stationary. That is, in general, PSF ({right arrow over (r)}), where {right arrow over (r)} is the location of the detection (e.g., conebeam or other 3-D-conversion collimator). Thus, a simple rebinning and subsequent use of 3D-modelling with iterative method (e.g., OSEM-3D) will not be accurate.
Therefore, there is a need for new and improved systems and methods for the rebinning of projection data using different collimators.